1. Field of the Invention
The present invention relates to a projection exposure apparatus for use to form a pattern of a semiconductor integrated circuit, or a liquid crystal device, or the like.
2. Related Background Art
When a circuit pattern of a semiconductor device or the like is formed, so-called photolithography technology is required. In this process, a method, in which a reticle (a mask) pattern is formed on a substrate such as semiconductor wafer, is usually employed. The surface of the substrate is applied with photosensitive photoresist so that a circuit pattern is transferred to the photoresist in accordance with an image irradiated with light, that is, in accordance with the shape of the pattern corresponding to a transparent portion of the reticle pattern. In a projection exposure apparatus (for example, a stepper), the image of a circuit pattern drawn on the reticle so as to be transferred is projected on the surface of the substrate (wafer) via a projection optical system so as to be imaged.
In an irradiation optical system for irradiating the reticle with light, an optical integrator such as a fly-eye type optical integrator (a fly-eye lens) and a fiber is used so as to uniform the distribution of the intensities of irradiation light with which the surface of the reticle is irradiated. In order to make the aforesaid intensity distribution uniform optimally, a structure which employs the fly-eye lens is arranged in such a manner that the reticle-side focal surface (the emission side) and the surface of the reticle (the surface on which the pattern is formed) hold a substantially Fourier transformed relationship. Also the focal surface adjacent to the reticle and the focal surface adjacent to the light source (the incidental side) hold the Fourier transformed relationship. Therefore, the surface of the reticle, on which the pattern is formed, and the focal surface of the fly-eye lens adjacent to the light source (correctly, the focal surface of each lens of the fly-eye lens adjacent to the light source) hold an image formative relationship (conjugated relationship). As a result of this, irradiation light beams from respective optical elements (a secondary light source image) of the fly-eye lens are added (superposed) because they pass through a condenser lens or the like so that they are averaged on the reticle. Hence, the illuminance uniformity on the reticle can be improved. Incidentally, there has been disclosed an arrangement capable of improving the illuminance uniformity in U.S. Pat. No. 4,497,015 in which two pairs of optical integrators are disposed in series.
In a conventional projection exposure apparatus, the light quantity distribution of irradiation beams to be incident on the optical integrator, such as the aforesaid fly-eye lens, has been made to be substantially uniform in a substantially circle area (or in a rectangular area), the center of which is the optical system of the irradiation optical system.
FIG. 14 illustrates a schematic structure of a conventional projection exposure apparatus (stepper) of the above described type. Referring to FIG. 14, irradiation beams L140 pass through a fly-eye lens 41c, a spatial filter (an aperture diaphragm) 5a and a condenser lens 8 so that a pattern 10 of a reticle 9 is irradiated with the irradiation beams L140. The spatial filter 5a is disposed on, or adjacent to a Fourier transformed surface 17 (hereinafter abbreviated to a xe2x80x9cpupil surfacexe2x80x9d) with respect to the reticle side focal surface 414c of the fly-eye lens 41c, that is, with respect to the reticle pattern 10. Furthermore, the spatial filter 5a has a substantially circular opening centered at a point on optical axis AX of a projection optical system 11 so as to limit a secondary light source (plane light source) image to a circular shape. The irradiation light beams, which have passed through the pattern 10 of the reticle 9, are imaged on a resist layer of a wafer 13 via the projection optical system 11. In the aforesaid structure, the number of apertures of the irradiation optical system (41c, 5a and 8) and the number of reticle-side apertures formed in the projection optical system 11, that is "sgr" value is determined by the aperture diaphragm (for example, by the diameter of an aperture formed in the spatial filter 5a), the value being 0.3 to 0.6 in general.
The irradiation light beams L140 are diffracted by the pattern 10 patterned by the reticle 9 so that 0-order diffracted light beam Do, +1-order diffracted light beam Dp and xe2x88x921-order diffracted light beam Dm are generated from the pattern 10. The diffracted light beams Do, Dp and Dm, thus generated, are condensed by the projection optical system 11 so that interference fringes are generated. The interference fringes, thus generated, correspond to the image of the pattern 10. At this time, angle xcex8 (reticle side) made by the 0-order diffracted light beam Do and xc2x11-order diffracted light beams Dp and Dm is determined by an equation expressed by sin xcex8=xcex/P (xcex: exposure wavelength and P: pattern pitch).
It should be noted that sin xcex8 is enlarged in inverse proportion to the length of the pattern pitch, and therefore if sin xcex8 has become larger than the number of apertures (NAR) formed in the projection optical system 11 adjacent to the reticle 9, the xc2x11-order diffracted light beams Dp and Dm are limited by the effective diameter of a pupil (a Fourier transformed surface) 12 in the projection optical system 11. As a result, the xc2x11-order diffracted light beams Dp and Dm cannot pass through the projection optical system 11. At this time, only the 0-order diffracted light beam Do reaches the surface of the wafer 13 and therefore no interference fringe is generated. That is, the image of the pattern 10 cannot be obtained in a case where sin xcex8 greater than NAR. Hence, the pattern 10 cannot be transferred to the surface of the wafer 13.
It leads to a fact that pitch P, which holds the relationship sin xcex8=xcex/P=NAR, has been given by the following equation.
P=xcex/NARxe2x80x83xe2x80x83(1)
Therefore, the minimum pattern size becomes about 0.5xc2x7xcex/NAR because the minimum pattern size is the half of the pitch P. However, in the actual photolithography process, some considerable amount of focal depth is required due to an influence of warp of the wafer, an influence of stepped portions of the wafer generated during the process and the thickness of the photoresist. Hence, a practical minimum resolution pattern size is expressed by kxc2x7xcex/NAR, where k is a process factor which is about 0.6 to 0.8. Since the ratio of the reticle side number of articles NAW and the wafer side number of articles NAR is the same as the imaging magnification of the projection optical system, the minimum resolution size on the reticle is kxc2x7xcex/NAR and the minimum pattern size on the wafer is kxc2x7xcex/NAW=kxc2x7xcex/Bxc2x7NAR (where B is an imaging magnification (contraction ratio)).
Therefore, a selection must be made whether an exposure light source having a shorter wavelength is used or a projection optical system having a larger number of apertures is used in order to transfer a more precise pattern. It might, of course, be considered feasible to study to optimize both the exposure wavelength and the number of apertures.
However, it is so far difficult for the projection exposure apparatus of the above described type to shorten the wavelength of the irradiation light source (for example, 200 nm or shorter) because a proper optical material to make a transmissive optical member is not present and so forth. Furthermore, the number of apertures formed in the projection optical system has approached its theoretical limit at present and therefore it is difficult to further enlarge the apertures. Even if the aperture can be further enlarged, the focal depth expressed by xc2x1xcex/2NA2 rapidly decreases with an increase in the number of apertures, causing a critical problem to take place in that the focal depth required in a practical use further decreases.
In Japanese Patent Publication No. 62-50811 for example, there has been disclosed a so-called phase shift reticle arranged in such a manner that the phase of each of transmissive light beams traveled from specific points in the transmissive portions of the circuit pattern of the reticle is shifted by xcfx80 from the phase of transmissive light beams traveled from the other transmissive portions. By using a phase shift reticle of the type described above, a further precise pattern can be transferred.
However, the phase shift reticle has a multiplicity of unsolved problems because of a fact that the cost cannot be reduced due to its complicated manufacturing process and inspection and modification methods have not been established even now.
Hence, an attempt has been made as projection exposure technology which does not use the phase shift reticle and with which the transference resolving power can be improved by modifying the method of irradiating the reticle with light beams. One irradiation method of the aforesaid type is a so-called annular zone irradiation method, for example, arranged in such a manner that the irradiation light beams which reach the reticle 9 are given a predetermined inclination by making the spatial filter 5a shown in FIG. 14 an annular opening so that the irradiation light beams distributed around the optical axis of the irradiation optical systemare cut on the Fourier transformed surface 17.
In order to establish projection exposure having a further improved resolving power and a larger focal depth, an inclination irradiation method or a deformed light source method has been previously disclosed in PCT/JP91/01103 (filed on Aug. 19, 1991). The aforesaid irradiation method is arranged in such a manner that a diaphragm (a spatial filter) having a plurality (two or four) openings, which are made to be eccentric with respect to the optical axis of the irradiation optical system by a quantity corresponding to the precision (the pitch or the like) of the reticle pattern, is disposed adjacent to the emission side focal surface of the fly-eye lens so that the reticle pattern is irradiated with the irradiation light beams from a specific direction while inclining the light beams by a predetermined angle.
However, the above mentioned inclination irradiation method and the deformed light source method have a problem in that it is difficult to realize a uniform illuminance distribution over the entire surface of the reticle because the number of effective lens elements (that is, the number of secondary light sources capable of passing through the spatial filter) decreases and therefore an effect of making the illuminance uniform on the reticle deteriorates. What is worse, the light quantity loss is excessive large in the system which has a member, such as the spatial filter, for partially cutting the irradiation light beams. Therefore, the illumination intensity (the illuminance) on the reticle or the wafer can, of course, deteriorate excessively, causing a problem to take place in that the time taken to complete the exposure process becomes long with the deterioration in the irradiation efficiency. Furthermore, a fact that light beams emitted from the light source concentrically pass through the Fourier transformed plane in the irradiation optical system will cause the temperature of a light shielding member, such as the spatial filter, to rise excessively due to its light absorption and a measure (air cooling or the like) must be taken to prevent the performance deterioration due to change in the irradiation optical system caused from heat.
In a case where a diaphragm of the aforesaid type is disposed adjacent to the emission side focal surface of the fly-eye lens, some of the secondary light source images formed by a plurality of the lens elements are able to superpose on the boundary portion between the light transmissive portion of the diaphragm and the light shielding portion of the same. This means a fact that the secondary light source image adjacent to the aforesaid boundary portion is shielded by the diaphragm or the same passes through the boundary portion on the contrary. That is, an unstable factor, such as the irradiation light quantity, is generated and another problem arises in that the light quantities of the light beams emitted from the aforesaid diaphragm and that are incident on the reticle become different from one another. Furthermore, in the inclination irradiation method, the positions of the four openings (in other words, the light quantity distribution in the Fourier transformed surface) must be changed in accordance with the degree of precision of the reticle pattern (the line width, or the pitch or the like). Therefore, a plurality of diaphragms must be made to be exchangeable in the irradiation optical system, causing a problem to arise in that the size of the apparatus is enlarged.
When a secondary light source formed on the reticle side focal surface of the fly-eye lens is considered in a case where the light source comprises a laser such as an excimer laser having a spatial coherence, the irradiation light beams corresponding to the lens elements have some considerable amount of coherence from each other. As a result, random interference fringes (speckle interference fringes) are formed on the surface of the reticle or the surface of the wafer which is in conjugate with the surface of the reticle, causing the illuminance uniformity to deteriorate. When its spatial frequency is considered here, a Fourier component corresponding to the minimum interval between the lens elements is present in main. That is, the number of combinations of light beams contributing to the interference is the largest. Therefore, fringes having a relatively low frequency (having a long pitch) in comparison to the limit resolution and formed to correspond to the configuration direction of the lens elements are observed on the surface of the reticle or the surface of the wafer. Although the formed interference fringes have low contrast because the KrF excimer laser has a relatively low spatial coherence, the interference fringe acts as parasite noise for the original pattern. The generation of the interference fringes causes a problem when the illuminance uniformity, which will be further required in the future, is improved. In the case where the annular zone irradiation method is considered, the aforesaid noise concentrically superposes in the vicinity of the limit resolution, and therefore the influence of the noise is relatively critical in comparison to the ordinary irradiation method (see FIG. 14).
An object of the present invention is to provide a projection exposure apparatus capable of obtaining high resolution and a large focal depth and revealing excellent illuminance uniformity even if an ordinary reticle is used.
In the present invention, the emission side focal surface is disposed on a Fourier transformed surface 17 with respect to a mask in the optical path of the irradiation optical system or on a plane adjacent to the same as shown in FIG. 1. Furthermore, there are a plurality of first fly-eye lenses 41a and 41b the centers of which are disposed at a plurality of positions which are eccentric from optical axis AX of the irradiation optical system, a plurality of second fly-eye lenses 40a and 40b having the emission side focal plane located on the Fourier transformed surface with respect to each incidental end of a plurality of the first fly-eye lenses 41a and 41b or on a surface adjacent to the same and disposed to correspond to the first fly-eye lenses 41a and 41b and light dividers for dividing the irradiation light beams from the light source to be incident on a plurality of the second fly-eye lenses 40a and 40b. Furthermore, a guide optical element is disposed so as to cause the light beams emitted from one of a plurality of the second fly-eye lenses to be incident on one of a plurality of the first fly-eye lenses. In a case where a laser represented by an excimer laser is used as the light source, an optical path difference generating member 70 is disposed between a plurality of the light beams emitted from the light dividers 20 and 21 shown in FIG. 17, the optical path difference generating member 70 causing an optical path difference (the phase difference) longer than the coherent distance (the coherent length) of the irradiation light beams to be given.
As shown in FIGS. 24 and 27, the present invention comprises, in an irradiation optical path, a plane light source forming optical system 100 or 106 and 107 for forming a plurality of light sources, a converging optical system 102 or 108 for converging the light beams from the plane light source forming optical system, a polyhedron light source forming optical system 103 having a plurality of lens elements 103a to 103d for forming a plurality of plane light source images on the Fourier transformed surface with respect to the reticle by the light beams from the converging optical system or on a plane adjacent to the same and having the centers of the optical axes disposed at a plurality of positions which are eccentric from the optical axis of the irradiation optical system, and a condenser for converging the light beams from the plurality of plane light source images formed by the polyhedron light source forming optical system onto the reticle.
In the aforesaid basic structure, assuming that half of the distance between the optical axes of the lens elements in a direction of the pattern of said reticle is L, the focal distance on the emission side of said condenser lens is f, the wavelength of said irradiation light beams is xcex and the cyclic pitch of said pattern of said mask is P, it is preferable to arrange the structure to satisfy the following condition:
L=xcexf/2P
In a case where the reticle has a two-dimensional pattern, the polyhedron light source forming optical system is composed of four lens elements disposed in parallel and, assuming that the number of apertures on the reticle side of said projection optical system is NAR, half of the distance between the optical axes of said lens elements 103a to 103d in a direction of the pattern of the reticle is L, and the emission side focal distance of the condenser lens 8 is f, it is preferable that the following conditions are satisfied:
0.35NARxe2x89xa6L/fxe2x89xa60.7NAR
As shown in FIG. 29, the present invention comprises light dividers 200 and 201 for dividing the irradiation light beams in the optical path of the irradiation optical system, polyhedron light source forming optical systems 202a, 202b, 203a, 203b, 204a and 204b for forming a plurality of plane light sources which correspond to each light beam divided by the light dividers on the Fourier transformed surface with respect to the reticle 9 or on a plane adjacent to the same at a plurality of positions which are eccentric from the optical axis of the irradiation optical system and a condenser lens 8 for converging the light beams from a plurality of the plane light sources onto the reticle, wherein the polyhedron light source forming optical system includes at least rod type optical integrators 203a and 203b. 
In the aforesaid basic structure, the polyhedron light source forming optical system may have a plurality of rod type optical integrators the centers of which are disposed at a plurality of positions which are eccentric from the optical axis of the irradiation optical system.
Furthermore, the polyhedron light source forming optical system may comprise a first converging lens for converging light beams divided by the light dividing optical system, a rod type optical integrator having the incidental surface disposed at the focal point of the converging lens and a second converging lens for converging the light beams from the rod type optical integrator to form a plurality of plane light sources on the Fourier transformed surface with respect to the reticle or on a plane adjacent to the same.
The operation of the present invention will now be described with reference to FIG. 13. The description will be given hereinafter about an example of the projection exposure apparatus in which the fly-eye type optical integrator (fly-eye lens) is disposed in the irradiation optical system. Referring to FIG. 13, second fly-eye lens groups 40a and 40b corresponding to the second fly-eye lens according to the present invention are disposed on a plane perpendicular to optical axis AX. Light beams emitted from them are incident on first fly-eye lens groups 41a and 42b, which correspond to the first fly-eye lens according to the present invention, by guide optical systems 42a and 42b. The illuminance distribution on the incidental surface of the first fly-eye lens is made uniform by the second fly-eye lens group.
Light beams emitted from the first fly-eye lens group are applied to a reticle 9 by a condenser lens 8. The illuminance distribution on the reticle 9 is made to be uniform by the first and the second fly-eye lens groups to a satisfactory degree. Reticle side focal surfaces 414a and 414b of the first fly-eye lens groups 41a and 41b substantially coincide with a Fourier transformed surface 17 of the reticle pattern 10. Therefore, the distance from optical axis AX to the center of the first fly-eye lens corresponds to the incidental angle of the light beams emitted from the first fly-eye lens on the reticle 9.
A circuit pattern 10 drawn on the reticle (the mask) includes a multiplicity of cyclic patterns. Therefore, the reticle pattern 10 irradiated with the irradiation light beams emitted from one fly-eye lens group 41a generates a 0-order diffracted light beam component Do, xc2x11-order diffracted light beam components Dp and Dm and higher diffracted light beam components in a direction corresponding to the precision of the pattern.
At this time, since the irradiation light beams (the main beams) are incident on the reticle while being inclined, also the diffracted light beam components are generated from the reticle pattern 10 while being inclined (having an angular deviation) in comparison to a case where the reticle 9 is irradiated perpendicularly. Irradiation light beam L130 shown in FIG. 13 is incident on the reticle 9 while being inclined by xcfx86 from the optical axis.
Irradiation light beam L130 is diffracted by the reticle pattern 10 and the 0-order diffracted light beam Do travelling in a direction inclined by xcfx86 from optical axis AX, +1-order diffracted light beam Dp inclined from the 0-order diffracted light beam by xcex8p and the xe2x88x921-order diffracted light beam Dm travelling while being inclined from the 0-order diffracted light beam Do by xcex8m are generated. However, since irradiation light beam L130 is incident on the reticle pattern while being inclined from optical axis AX of the double telecentric projection optical system 11 by an angle xcfx86, also the 0-order diffracted light beam Do also travels in a direction inclined by an angle xcfx86 from optical axis of the projection optical system.
Therefore, the +1-order diffracted light beam Dp travels in a direction xcex8p+xcfx86 with respect to optical axis AX, while the xe2x88x921-order diffracted light beam Dm travels in a direction xcex8mxe2x88x92xcfx86 with respect to optical axis AX.
At this time, the diffracted angles xcex8p and xcex8m respectively are expressed by:
sin(xcex8p+xcfx86)xe2x88x92sin xcfx86=xcex/Pxe2x80x83xe2x80x83(2)
sin(xcex8mxe2x88x92xcfx86)+sin xcfx86=xcex/Pxe2x80x83xe2x80x83(3)
Assumption is made here that both of the +1-order diffracted light beam Dp and the xe2x88x921-order diffracted light beam Dm pass through a pupil surface 12 of the projection optical system 11.
When the diffraction angle is enlarged with the precision of the reticle pattern 10, the +1-order diffracted light beam Dp travelling in the direction xcex8p+xcfx86 cannot pass through the pupil 12 of the projection optical system 11. That is, a relationship expressed by sin(xcex8p+xcfx86) greater than NAR is realized. However, since irradiation light beam L130 is incident while being inclined from optical axis AX, the xe2x88x921-order diffracted light beam Dm is able to pass through the projection optical system 11 at the aforesaid diffraction angle. That is, a relationship expressed by sin(xcex8mxe2x88x92xcfx86) less than NAR is realized.
Therefore, interference fringes are generated on the wafer due to the 0-order diffracted light beam Do and the xe2x88x921-order diffracted light beam Dm. The aforesaid interference fringes are the image of the reticle pattern 10. When the reticle pattern is formed into a line-and-space pattern having a ratio of 1:1, the image of the reticle pattern 10 can be patterned on the resist applied on the wafer 13 while having a contrast of about 90%.
At this time, the resolution limit is present when the following relationship is realized:
sin(xcex8mxe2x88x92xcfx86)=NARxe2x80x83xe2x80x83(4)
Therefore, the pitch on the reticle side of the minimum pattern which can be allowed to be transferred can be expressed by:
NAR+sin xcfx86=xcex/P
P=xcex/(NAR+sin xcfx86)xe2x80x83xe2x80x83(5)
In a case where sin xcfx86 is made to be about 0.5xc3x97NAR, the minimum pitch of the pattern on the reticle which can be transferred becomes as follows:
P=xcex/(NAR+0.5NAR)=2xcex/3NARxe2x80x83xe2x80x83(6)
In a case of a conventional exposure apparatus shown in FIG. 14 in which the irradiation light beam distribution on the pupil 17 is in a circular region relative to optical axis AX of the projection optical system 11, the resolution light is P=xcex/NAR as expressed by Equation (1). Therefore, the present invention enables a higher resolution in comparison to the conventional exposure apparatus.
Now, the description will be given about the reason why the focal depth can be enlarged by irradiating the reticle pattern with exposure light beams from a specific incidental direction and at a specific angle by a method in which the image pattern is formed on the wafer by using the 0-order diffracted light beam component and the 1-order diffracted light beam component.
In a case where the wafer 13 coincides with the focal point position (the best imaging surface) of the projection optical system 11, the diffracted light beams emitted from a point of the reticle pattern 10 and reaching a point on the wafer have the same optical path length regardless of the portion of the projection optical system 11 through which they pass. Therefore, even in the conventional case where the 0-order diffracted light beam component passes through substantially the center (adjacent to the optical axis) of the pupil surface 12 of the projection optical system 11, optical length for the 0-order diffracted light beam component and that for the other diffracted light beam component are substantially the same and the mutual wavelength aberration is zero. However, in a defocus state in which the wafer 13 does not coincide with the focal point position of the projection optical system 11, the optical path length for a higher diffracted light beam made incident diagonally becomes short in front of the focal point in comparison to the 0-order diffracted light beam which passes through a portion adjacent to the optical axis and as well as lengthened in the rear of the focal point (toward the projection optical system 11) by a degree corresponding to the difference in the incidental angle. Therefore, the diffracted light beams such as 0-order, 1-order and higher order diffracted light beams form mutual wave aberration, causing an out of focus image to be generated in front or in the rear of the focal point position.
The wave aberration generated due to the defocus is a quantity given by xcex94Fr2/2 assuming that the quantity of deviation from the focal point position of the wafer 13 is xcex94F and the sine of incidental angle xcex8w made when each diffracted light beam is incident on one point of the wafer is r (r=sin xcex8w), where r is the distance between each diffracted light beam and optical axis AX on the pupil surface 12. In the conventional projection exposure apparatus shown in FIG. 14, the 0-order diffracted light beam Do passes through a position adjacent to the optical axis. Therefore, r (0-order) becomes 0, while xc2x11-order diffracted light beams Dp and Dm hold a relationship expressed by r (1-order)=Mxc2x7xcex/P (where M is the magnification of the projection optical system). Therefore, the wave aberration between the 0-order diffracted light beam Do and xc2x11-order diffracted light beams Dp and Dm becomes:
xcex94Fxc2x7M2(xcex/P)2/2
In the projection exposure apparatus according to the present invention, the 0-order diffracted light component Do is generated in a direction inclined from optical axis AX by an angle xcfx86 as shown in FIG. 13. Therefore, the distance between the 0-order diffracted light beam component and the optical axis AX on the pupil surface 12 holds a relationship expressed by r (0-order)=Mxc2x7sin xcfx86.
The distance between the xe2x88x921-order diffracted light beam component and the optical axis on the pupil surface becomes a value obtainable from r (xe2x88x921-order)=Mxc2x7sin(xcex8mxe2x88x92xcfx86). If sin xcfx86=sin(xcex8mxe2x88x92xcfx86) at this time, the relative wave aberration between the 0-order diffracted light beam component Do and the xe2x88x921-order diffracted light beam component Dm due to defocus becomes zero. Hence, even if the wafer 13 is slightly deviated in the direction of the optical axis from the focal point position, the out of focus of the image of the pattern 10 can be prevented. That is, the focal depth can be enlarged. Furthermore, since sin(xcex8mxe2x88x92xcfx86)+sin xcfx86=xcex/P as expressed by the equation (3), the focal depth can be significantly enlarged by causing the incident angle xcfx86 for the irradiation light beam L130 on the reticle 9 to hold a relationship expressed by sin xcfx86=xcex/2P with the pattern having pitch P.
In the present invention, the irradiation light beams emitted from the light source are divided into a plurality of light beams before they are introduced into each fly-eye lens. Therefore, the light beams emitted from the light source can be efficiently utilized while reducing loss, so that a projection exposure system revealing high resolution and a large focal depth can be realized.
As described above, according to the present invention, the necessity lies in simple fact that the irradiation optical system of the projection exposure apparatus which is being operated is changed at the manufacturing process. Therefore, the projection optical system of an apparatus which is being operated can be utilized as it is and further improved resolution and dense integration can be realized.
Although the irradiation system for use in the present invention becomes complicated in comparison to an ordinary system, the uniformity of the illuminance on the reticle surface and on the wafer surface can be improved because the fly-eye lenses are disposed to form two stages in the direction of the optical axis. By virtue of the two stage fly-eye lens structure, the illuminance uniformity on the reticle and the wafer surfaces can be maintained even if the fly-eye lens is moved on a plane perpendicular to the optical axis.
Since the light dividing optical system efficiently introduces the irradiation light beams to the first stage fly-eye lens, the irradiation light quantity loss can be satisfactorily prevented. Therefore, the exposure time can be shortened and the processing performance (throughput) cannot deteriorate.
In a system in which the second stage fly-eye lens adjacent to the reticle is made movable as in an embodiment (see FIG. 5), optimum irradiation can be performed in accordance with the reticle pattern.
In a system arranged in such a manner that the first, the second fly-eye lenses and the guide optical system are integrally held while making them to be movable, the movable portion can be decreased and therefore the structure can be simplified. As a result, the manufacturing and adjustment cost can be reduced.
Also in a case where a plurality of the guide optical system and the corresponding first fly-eye lens are respectively made movable, the light dividing optical system and the second fly-eye lens group are integrally held. Therefore, the structure can be simplified and as well as the manufacturing cost and the adjustment cost can be reduced.
In a system in which the light dividing optical system or a portion of the same is made to be movable, the optimum dividing optical system (dividing into two portions and that into four portions can be selected) can be used in accordance with the division conditions.
In a system in which at least a portion of the light dividing optical system can be moved or rotated, the condition of dividing the light beams can be varied by, for example, changing the interval between the polyhedron prisms or by rotating the polyhedron prism. Therefore, a variety of division states can be created by using a small number of optical members.
Also in a case where a rod type optical integrator is used in place of the fly-eye type optical integrator (the fly-eye lens), or in a case where they are combined to each other, an effect similar to the aforesaid structures can be obtained.
Furthermore, the present invention is arranged in such a manner that the irradiation light beams emitted from the light source are divided into a plurality of light beams before a phase difference (the difference in the length of the optical path) which is longer than the coherent distance (coherent length) of the irradiation light beams is given to a portion between a plurality of the light beams. The coherent length LS of the irradiation light beam can be expressed by:
LS=xcex2/D1
(where the wave length of the irradiation light beam is xcex and its vector width is D1).
That is, if a difference in the optical path length longer than the coherent length L is present between two light beams emitted from one light source, the two light beams do not interfere with each other. In a case where the light source is a narrow band KrF excimer laser, the coherent length L is about 20 mm and therefore an optical path difference can be relatively easily given to a plurality of light beams. Therefore, even if a laser having a certain coherence is used, the speckle interference fringe which can be superposed on the desired pattern as noise can be effectively reduced. That is, the illuminance uniformity on the reticle and the wafer can be improved by necessitating. a simple structure in which the optical path difference generating member is disposed in the irradiation optical path.
Other and further objects, features and advantages of the invention will be appeared more fully from the following description.